1. Field of the Invention
This invention relates to a frequency hopping communication method and device, especially to sequentially changing carrier frequencies of transmitted data according to a frequency hopping spread code list. This is applicable to a spread spectrum communication using frequency hopping methods.
2. Description of the Prior Art
Spread spectrum communication is a communication method by spreading signals in bands frequency broader than the data to be transmitted. Its merits are, for example, high tolerance of interference, confidentiality, and applicability to high resolution distance measuring. It is used conventionally for satellite and ground communication. Recently, it is also being applied to mobile car communication and intercommunication (local area network communication), since it is expected to improve communication capacity for the same frequency band, and since it can co-exist with pre-existing systems.
Representative methods to implement spread spectrum communication include Direct Sequence (DS) and Frequency Hopping (FH) methods. DS methods spread the utilized frequency band by balance modulation of spread code pulses directly on data modulated by carriers. On the other hand, FH methods use broad frequency bands by changing (i.e., hopping) carrier frequencies of modulated data using spread code pulses. Particularly, by changing frequencies faster than information change speed, Fast Frequency Hopping (FHH) is more tolerant of interference, shows fewer far-to-near problems or frequency diversity effects, and is attractive for either mobile communication or indoor communication where the phasing effect is great.
In a normal FH method, generally speaking, Frequency Shift Keying (FSK) is used for modulating data. In other words, each several bits of data to be transmitted is converted into code words, and frequencies are shifted in accordance with codes (code word chips) which make code words. FIG. 1 shows a relationship between bit strings of data and code words being converted. As is shown in FIG. 1, every three bits of input data is converted into any of eight code word patterns. For example, if input data is 000, it is converted into 7-6-5-2-4-1-3. The integers "0" through "7" which constitute code words are simply called codes or code word chips. Their arrangement in each code word is designed so that a receiver can distinguish data of "000" through "111".
Different frequencies are assigned to each code. For example, frequencies of f0 through f7 correspond to codes of 0 through 7 one to one. If "000" is transmitted, frequencies will be changed in a sequence of f7, f6, f5, f2, f4, f1, f3 corresponding to code words 7-6-5-2-4-1-3. Since there are eight frequencies to be used, this modulation is called eight level MFSK (Multilevel FSK) in this case. Generally speaking, if there are 2.sup.n modulation levels, the number of codes comprising one code word is less than 2.sup.n -1. Hereafter, modulation of data (modulation which has nothing to do with frequency hopping) is called primary modulation.
On the other hand, frequency hopping of carriers is done according to frequency hopping spread code sequence (i.e., pulse strings of pseudo noise code. Hereafter, it is simply called "code sequence".). For example, if there are 31 codes in the code sequence, there will be 31 choices of frequencies as hopping frequencies within a frequency band usable for communication. (Frequency hopping itself is included in FSK in a broader sense. However, in this description, FSK only means primary modulation.) A period in which a code sequence loops is called a code period, and a period in which a hopping frequency changes (in this case, 1/31 of the code period) is called a hopping period. Changing of hopping frequencies and frequency change by primary modulation are accomplished synchronously.
This kind of FH-MFSK method has up to this time been introduced in various publications as a highly harmonious combination. One representative example is "Frequency-Hopped Multilevel FSK for Mobile Radio" by J. D. Goodman et. al., The Bell System Technical Journal, Vol. 59 No. 7 p1257.about.1275, 1980. Furthermore, a combination method of M-ary FSK and FH is described in "Concept and Basic Characteristics of Experimental Device for Mobile Communication of Frequency Hopping" by E. Morimatsu et. al., Radio Wave Research Institute seasonal report Vol. 32, No. 164 p165.about.177, 1986.
Frequency methods, especially the FFH method, have general problems in synchronization acquisition by a frequency synthesizer in a receiver, and in keeping accurate synchronization, as well as in complicated hardware resulted from these. Therefore, coherent reverse spread method by delay correlation has been proposed as an easier method for receiving data.
Japanese Patent Laid-Open Publication No. Hei 03-212037 discloses a revised delay correlation method taking interference by other stations into consideration. In this method, the code sequence is divided into a former half and a latter half. The code sequence of the former half is modulated by data to be transmitted, and that of the latter half is modulated and transmitted by predetermined intermediate frequency signals. The receiver measures correlation of timing delay by a half of the code sequence. It extracts signal components in narrow band centered at a fixed intermediate frequency of the transmitter, and then demodulates data bits. FIGS. 2(A) through 2(E) show a code sequence and data demodulated by the receiver if the number of codes N=8. FIG. 2(A) shows a single sequence in which the latter four codes of a code sequence are shifted by an intermediate frequency fx. FIG. 2(B) shows four data values a-d to be conveyed upon the former four codes. FIG. 2(C) shows a signal sequence obtained by modulating the signal shown in FIG. 2(A) by the data shown in FIG. 2(B). FIGS. 2(D) and 2(E) show a signal sequence when demodulated by delay correlation performed by the receiver. Here, by mixing modulation signal (C) and signal (D) which is delayed by Td/2, carrier frequencies f1 through f4 are eliminated, and synthetics of the intermediate frequency and data are demodulated. In the same bulletin, it is stated that data signals can be transmitted not only by frequency modulation but also by phase modulation. As described above, interference by other stations can be decreased by allocating different intermediate frequencies to each station.
However, transmission efficiency is decreased in the above-described method, since information transmission is done for only half of the code sequence. Decreased transmission efficiency is obviously one of the most serious problems in communication.
On the other hand, in the FH-MFSK method, transmission efficiency will be raised by increasing the available frequencies for MFSK (i.e., level numbers in multilevel). However, increasing level numbers means wider frequency band to be used in MFSK. Since today a variety of communication devices use a plurality of frequency bands, available frequency bands for each communication method or communication device is regulated by laws regarding radio waves. In Japan, concerning a spread spectrum communication of FH-MFSK method, only a frequency band of 2471.about.2497 MHz is available for specific low-power data communication. A way of improving communication capacity for the same frequency band without raising MFSK level numbers is desired.